Example case:
- gem install (.circleci/config.yml)
Example errors:
- ruby writing permission exhibited


- invalid ruby version

Solution:
Customize ruby version in .circleci/config.yml:

Reference:
CircleCI 2.0 > Testing iOS Applications on macOS > Using Custom Ruby Versions. https://circleci.com/docs/2.0/testing-ios/#using-custom-ruby-versions